For measuring electrically conductive fluids, flow meters using a magneto-inductive flow pickup are often employed. In the following, when necessary, for the sake of simplicity, discussion will only be in terms of flow pickups, or flow meters, as the case may be. As is known, magneto-inductive flow meters permit measurement of the volume flow rate of an electrically conductive liquid flowing in a pipeline and the reflecting of such in a corresponding, measured value; thus, the volume of liquid flowing through a pipe cross section per unit time is measured. Construction and operation of magneto-inductive flow meters are known, per se, to those skilled in the art and are described extensively and in detail in DE-A 43 26 991, EP-A 1 275 940, EP-A 1 273 892, EP-A 1 273 891, EP-A 814 324, EP-A 770 855, EP-A 521 169, U.S. Pat. Nos. 6,031,740, 5,487,310, 5,210,496, 4,410,926, US-A 2002/0117009, or WO-A 01/90702.
Flow pickups of the described kind usually each have a non-ferromagnetic measuring tube, which is inserted fluid-tightly into the pipeline, e.g. by means of flanges or screwed connections. An electric voltage produced by charge separations in the flowing fluid by means of a magnetic field directed transversely to a flow direction of the fluid to be measured is sensed as a measured voltage by means of at least two measuring electrodes and further processed in the measuring device electronics to a corresponding, measured value, for example a measured value of volume flow rate. The part of the measuring tube contacting the fluid is, in general, electrically non-conductive, in order that the measured voltage, induced in the fluid according to Faraday's Law of Induction by the magnetic field passing, at least in part, through the measuring tube, not be short circuited. Metal measuring tubes are, therefore, usually provided internally with an electrically non-conducting liner, e.g. of hard rubber, polyfluoroethylene, etc., and are also, in general, non-ferromagnetic; in contrast, in the case of measuring tubes made completely of a plastic or a ceramic, especially aluminum oxide ceramic, the electrically non-conducting liner is not required.
In the case of magneto-inductive flow pickups, the magnetic field required for the measuring is produced by a corresponding magnetic field system, which contains a coil arrangement having an inductance L and, most often, at least two field coils, corresponding coil cores and/or pole shoes for the field coils and, as required, magnetically conductive, field-guiding sheets connecting the coil cores outside of the measuring tube. However, also magnetic field systems with a single field coil are known. The coil cores and/or pole shoes of the magnetic system are, most often, made of a soft-magnetic material. Magnetic systems with ferromagnetic coil cores have, however, also already been disclosed. The magnetic field system is usually arranged directly on the measuring tube.
For producing the magnetic field, a coil current delivered from a corresponding measuring device electronics is caused to flow in the coil arrangement. In order that the magnetic field produced by the magnetic field system be as homogeneous as possible, the field coils are, in the most frequent and simplest case, identical to one another and connected electrically in the same sense in series, so that the same exciter current can flow through both coils during operation. It has, however, also already been disclosed to cause an exciter current to flow through the coils alternately, first with the same sense, and then with opposite sense, in order, in this way, to be able to determine, for example, the viscosity of liquids and/or a degree of turbulence of the flow; see, in this connection, also EP-A 1 275 940, EP-A 770 855, DE-A 43 26 991. The aforementioned exciter current is produced by an operating electronics; it is regulated to a constant electrical current value of e.g. 85 mA, and its electrical current direction is periodically reversed. The electrical current reversal is achieved by placing the coils in a so-called T-circuit or in a so-called H-bridge circuit; on the subject of electrical current regulation and direction reversal, compare U.S. Pat. Nos. 4,410,926, or 6,031,740.
The coil current is, in the case of modern flow pickups, usually a strobed, bi-polar, rectangular, alternating current, which is positive in a first half-period of a period, with a constant, first electrical current end value and which then, by switching, is negative in a second half-period of the period, with a constant, second electrical current end value essentially equal in absolute value to that of the first electrical current end value. The coil arrangement can be e.g. a single coil, when the magneto-inductive flow pickup serves as a flow probe (compare U.S. Pat. No. 3,529,591), or it can also be composed e.g. of two coil portions, which are arranged diametrically opposite to one another on opposite sides of the measuring tube. In U.S. Pat. Nos. 6,763,729, 6,031,740 or 4,410,926, a circuit arrangement for producing such a coil current is described. This circuit arrangement includes an energy, or power, supply driving the coil current, as well as a bridge circuit, embodied as an H-bridge circuit, for modulating the coil current, with the coil arrangement lying in a transverse branch of the bridge circuit. Additionally, U.S. Pat. Nos. 6,031,740 and 4,410,926 show a circuit arrangement for producing a coil current, which, instead of the bridge circuit embodied as an H-bridge circuit, has a bridge circuit designed as a T-circuit for the coil arrangement. Furthermore, U.S. Pat. No. 4,204,240 describes a circuit arrangement with an internal energy supply for producing the coil current of a magneto-inductive flow pickup, which gives off a voltage, of which a voltage beginning value in each mentioned half-period is higher during a rising period of the coil current—as a first period portion of the half-period—than a voltage end value during a second period portion—as remainder of the half-period.
The mentioned, induced voltage arises between at least two galvanic (thus, wetted by the liquid), or between at least two capacitive (thus, e.g., embedded inside the wall of the measuring tube), measuring electrodes, with each of the electrodes sensing one potential. In the most frequent case, the measuring electrodes are so arranged lying diametrally opposite to one another that their common diameter lies perpendicular to the direction of the magnetic field and, therefore, perpendicular to the diameter on which the coil arrangements lie. The induced voltage is amplified and conditioned by means of an evaluating circuit to form a measurement signal, which is recorded, displayed, or itself processed further. Corresponding measurement electronics are likewise known to those skilled in the art, for example from EP-A 814 324, EP-A 521 169, or WO-A 01/90702.
Besides the actual measuring function, modern magneto-inductive flow meters often also include superordinated diagnosis functions, by means of which the flow meters can be subjected to a self-test during operation. Such diagnosis functions are described, for example, in the already mentioned U.S. Pat. No. 6,763,729, or EP-A 1 217 337. These self-tests can ascertain, on the one hand, whether all components of the measuring device are fully capable of performing their various functions, and, on the other hand, whether the measuring is proceeding as specified. If the result of such a self-test is positive, thus, if no malfunctions of the measuring device, or measuring operation, are present, then it is assured that the measurement result issued by the measuring device corresponds to the current, actual value, naturally, to within predetermined tolerances. Especially, individual components and parts of the measuring device can be reviewed, as to their ability to function, within the framework of such a self-test, e.g. by means of impedance measurements or measurements of ohmic resistance, or conductance, as the case may be. Furthermore, it is also possible e.g. to review the settings data at input as to plausibility. In such case, e.g. inputs lying outside of an allowable range are rejected and not transferred. Independently of the testing of the settings data at input, data calculated from the settings data, i.e. data which directly control the measuring operation of the measuring device, are checked again as to their allowed limits. This reviewing occurs before starting measuring operation, so that, in the case of an occurrence of an error with reference to the settings data, a corresponding error report can be issued and, in keeping with this, the measurement is not performed.
In the case of the measuring device described in EP-A 12 17 337, it is proposed to keep the exciter current variable in its electrical current strength, to measure the measurement voltage corresponding to the exciter current, and, on the basis of a deviation, beyond a threshold value, of the measured voltage levels, measured at various current strengths, from the voltage level expected for the measurement voltage on the basis of the functional relationship between the exciter current and the measurement voltage, a malfunctioning of the measurement operation of the measuring device can be detected. Thus, the functional relationship between the exciter current and the measurement voltage forming the basis for the measurement operation is assumed to be known and is used for the testing of the measurement operation. In the simplest case, this functional relationship between the exciter current and the measurement voltage can be linear, so that a malfunctioning of the measurement operation of the measuring device can be detected simply by a deviation of the measurement voltage levels from one another achieved at various current strengths by more than a threshold value from a linear curve.
In contrast, in the case of the flow meter described in U.S. Pat. No. 6,763,729, the testing determines, following the change of current direction, at least one parameter of the electrical current increase, and this is compared with a reference value. In such case, there is used as parameter a time span extending between two predetermined current values. Since the increase of the current satisfies a predetermined, physical law, which is, as a rule, an e-function, it is sufficient to determine the rise time between two values, in order to obtain a reliable statement concerning the electrical current increase. Alternatively, or in supplementation thereof, there can be used as parameter a time span extending between the switching of the current direction and the reaching of a predetermined electrical current value. The point in time of the switching must be determined very accurately. For example, the switching signal can be used as trigger signal for a time counter. The predetermined current value can lie, for example, in the vicinity of the maximum current value, i.e. in the vicinity of the electrical current strength, which is assumed following the settling of the magnetic field to a constant value suitable for the measuring. Additionally, in U.S. Pat. No. 6,763,729, an increased voltage is used following the switching. As already disclosed in U.S. Pat. No. 6,031,740, such an extra voltage accelerates the build-up of the magnetic field, since it increases the electrical current rise and thus makes it possible to repeat the actual measuring more quickly.
The threshold, or reference, value, in the case of the exceeding of which a malfunction of the measuring operation of the measuring device is certifiable, can be determined in the flow meter itself at an earlier point in time. For example, the desired parameter can be determined at the time of start-up and digitally stored as reference value in a microcomputer provided in the measuring electronics, so that it is available for future review procedures. In this way, each flow meter receives an individual reference value. The reference value for the deviation can be established in different ways, e.g. as a constant, absolute value. Additionally, it is possible to establish the threshold value as a percentage fraction of the measuring range end value for the measurement voltage. In this way, it can be achieved, that, by establishing the threshold value just once—namely as a percentage fraction of the measuring range end value for the initial size—a setting of the measuring device is made, which is applicable for all settable measuring ranges of the measuring device.
Once a malfunction of the measuring operation of the measuring device has been detected, various options are possible. For example, measuring operation of the measuring device can be ended, since, in such case, no further reliable measurement results can be obtained. Alternatively, or supplementally, once a malfunction of the measuring operation is detected, also optical and/or acoustical, warning reports can be issued.
Despite this method already known from the state of the art for self-test of measuring devices, there is still a need to provide other methods for self-test, with which to establish with yet greater accuracy, whether a malfunction of the measuring operation or of the measuring device itself is present. Especially, the case can arise, namely, that individual components of the measuring device only slightly deviate from their desired values, so that each individual component is noted within the framework of a self-test as fully capable of functioning, yet, in the interaction of the components, a measuring operation is generated, which is faulty, such that the stated accuracy of the measured values cannot be assured.
Moreover, it has been found that eddy currents induced in the magnet system, due to the switching and the rise of the coil current, prevent, as discussed also in U.S. Pat. No. 6,031,740, that the rise of the magnetic field exactly follows the rise of the coil current, such as would be the case in the absence of coil cores and/or pole shoes. Rather, the rise of the magnetic field is delayed and flattened, as compared with the coil current.
In the ideal case, the plot of current versus time for the current in the coil arrangement corresponds to the plot of the magnetic field versus time. Due to the eddy currents, which arise in the pole shoes and cores of the coil arrangement during the switching of the magnetic field, as a matter of fact, deviations from the ideal case occur. The coil current measured outside of the coil arrangement corresponds, therefore, to the sum of the electrical current flowing in the coil and the electrical current produced by the eddy currents. If the electrical current measured outside of the coil arrangement is used as the controlled variable, then, as a result, indeed, the current is held constant, but not the magnetic field. Such is true, so long as the eddy currents have not yet decayed.
This disadvantageous effect of the eddy currents occurs also in the case of, and despite, the mentioned increased voltage. The effect of the eddy currents can be illustrated by imagining that, connected in parallel with the (coil-) inductance, there is an eddy current source, whose current adds to the current in the inductance to make-up the total exciter current. Therefore, the voltage drop across a resistor lying in series with the inductance of the field coil is, indeed, a measure for the exciter current, but not, however, for the true field current in the inductance actually corresponding with the magnetic field. This is, however, necessary to know for an exact control of the magnetic field and, to such extent, also of advantage for the review of the magnetic field system.